Economic efficiency is an important consideration in the art of aircraft design. In recent times the environmental impact of the aircraft has also become an important factor included in the design process. In general it can be asserted that both economic and environmental efficiency are improved when the aircraft has a low fuel consumption. The main contributing factors to reduce the fuel consumption of an aircraft are: lower aerodynamic drag, lower structural weight and higher propulsive efficiency.
The aerodynamic drag of an aircraft can be interpreted as the energy per unit length that the aircraft transfers to the air in which it moves and is, in fact, the force opposing the movement of the aircraft which the thrust provided by the propulsive system must equate in steady and level flight.
Various physical phenomena contribute to the generation of aerodynamic drag giving rise to various forms of drag analysed in the aircraft design process, principally;                Friction drag, produced by the transfer of kinetic energy to the boundary layer or air that surrounds the skin of the aircraft and that becomes the wake of turbulent air that the vehicle leaves behind. Friction drag increases with the square of the velocity and is proportional to the wetted area, which is the surface area of the aircraft skin exposed to the external airflow. In order to reduce friction drag it is desirable to reduce the wetted area of the aircraft.        Induced drag or lift-induced drag is a drag force that occurs whenever a moving object of finite size redirects the airflow coming at it. This drag force typically occurs in aircraft due to wings redirecting the incoming air downwards to produce lift. With other parameters remaining the same, as the aircraft angle of attack increases, induced drag is also increased.        
The aircraft lift force is produced by accelerating the airflow over the upper surface of a wing, thereby creating a pressure difference between the air flowing over the wing upper and lower surfaces. On a wing of finite span, some air flows around the wingtip from the lower surface to the upper surface producing wingtip vortices which trail behind the aircraft wings. The kinetic energy absorbed by the wingtip vortices is ultimately extracted from the propulsive system of the aircraft and therefore is a form of drag. These wingtip vortices also modify the airflow around a wing, compared to a wing of infinite span, reducing the effectiveness of the wing to generate lift, thus requiring a higher angle of attack to compensate, and tilting the total aerodynamic force rearwards. Induced drag on airfoils is inversely proportional to the square of the airspeed, i.e., if the speed of the aircraft increases, the induced drag is reduced on airfoils as the total mass of air deflected by the wing per unit time is increased.
Induced drag depends, on one side, on the wing planform and, on the other side, on the aircraft speed. A high aspect ratio wing, i.e., a wing which is long and slender produces less induced drag. However, in these long and slender wings the lifting forces create large cantilevered loads and therefore large bending moments, especially at the wing roots, which lead to increased structural wing and aircraft weight.
The increased weight of slender wings led in the early days of aviation to aircraft comprising multiple airfoils braced by struts and cables, being a biplane design usual. As new materials became available, aircraft design developed into the monoplane configuration, with wing aspect ratios in the order of 10, as a compromise between low induced drag and acceptable structural weight.                Wave or Compressible drag. Modern high-speed aircraft cruise at speeds close to the speed of sound, at around Mach 0.8, i.e., eight tenths of the speed of sound. At these high speeds the airflow is accelerated by the shape of the airfoil which may lead to local flow velocities very close or above the speed of sound, which in turn produces a loss of kinetic energy due to irreversible effects in the compression and expansion of the air. This is another form of aerodynamic drag, particular of the flight at speeds close or above the speed of sound, known as wave or compressible drag due to compressible effects. It has been well known since the mid 20th century that the wave drag can be significantly decreased by designing the wings with sweepback so that the local airflow goes around an airfoil of an apparent thickness reduced by the cosine of the sweepback angle, whereas structurally the wing behaves as having its real thickness.        
As the aircraft must provide enough aerodynamic lift to sustain its weight in steady flight, it is clear that, for a given aircraft configuration and payload, heavier aircrafts will have more drag and thus more fuel consumption, being structural efficiency or lightness a desirable design feature in order to improve the economic efficiency of aircraft.
One measure of the overall propulsive efficiency of the powerplant of the aircraft is the mass of fuel required to provide a given thrust force per unit of time. For the thermal engines used in aeronautics, e.g., turbojets, turbofans, prop-fans, turboprops, piston engines etc. . . . , the overall propulsive efficiency depends on the design of the internal machinery and operating temperatures of the engine thermodynamic cycle but also inversely on the ratio of the velocity of the exhaust gases to the velocity of the aircraft. Therefore, in order to increase the propulsive efficiency of an aircraft engine it is desirable to increase the diameter of the elements that impart linear momentum to the air, e.g., propeller, fan, unducted-fan, so that for a given thrust force, i.e. momentum transfer per unit time, the mass flow is increased and the exhaust velocity is lowered. This has lead to a continuous increase in the diameter of aircraft engines during the past decades, to a point where it is becoming difficult to position the engines in the classical location under the wings.
An additional consideration regarding the environmental efficiency of an aircraft is the noise signature that it produces along its flight path, particularly in the take-off and landing phases, where the aircraft is closest to the ground. Increasing the diameter of the propulsive elements also helps to reduce the noise emitted by the engine. Additional perceived noise reductions can be obtained if the noise radiated by the engines can be shielded by the structure of the aircraft.
A typical modern large high speed transport aircraft tends to be of the monoplane configuration, with a single wing or airfoil of an aspect ratio around 10 and wing sweepback angles of around 30 to 40 degrees, with engines of large diameter hanging from under the wings or attached to the rear portion of the fuselage. This configuration has evolved during the last several decades and has become highly optimised. However, based on our previous discussion, it is evident that further improvements in terms of fuel consumption could be possible if the wing aspect ratio could be increased without an excessive weight penalty, or if the total wetted area of the aircraft could be reduced, for example removing stabilising elements in the empennage which do not contribute directly to the generation of lift. The overall propulsive efficiency could also be increased if the aircraft configuration could accommodate engines of larger diameter.
Likewise, a design improvement could be associated to a reduction of the perceived noise on the ground, either obtained by engines of larger diameter or by an aircraft configuration which helps to shield the engine noise from the ground.
Various inventors have contributed to the development of aircraft concepts that aspire to accomplish some of the aircraft design improvements listed above.
For example, document WO 2004/074093 discloses a swept-wing box-type aircraft comprising negative sweep wings connected to the fuselage rear upper portion, the positive sweep angle wings being connected to the fuselage forward lower portion, such that this wing configuration defines an aerodynamic channel intended to provide aircraft static flight stability. The merit of this configuration is that both wings contribute to the generation of lift, thereby removing the horizontal stabilising surfaces of the classical configuration, the said surfaces, although providing stability, contribute to increase the friction drag. Moreover, as the wings are joined at the tips, the tip vortices of each wing tend to cancel each other, which reduces the induced drag of the lifting system of airfoils. From the structural point of view, joining the wings at the tip provides mutual torsional support between the wings, which should tend to reduce the weight. However, this aircraft configuration, where the rear wing is higher than the forward wing, is prone to the well known problem of deep stall, in which the separated airflow from the fore wing at high angles of attack can blank the aft wing, leading to a stable and difficult to recover aircraft pitch-up attitude and loss of lift. Additionally, the engines are located in the fuselage, so that in cases where the aircraft is subjected to high accelerations, the inertial loads introduced by the engines will have to be transmitted by the fuselage to the wings, leading to increased weight. Moreover, the landing gear is also located in the lower portion of the fuselage, between the wings, so that in cases of landing with high vertical accelerations, the fuselage will have to resist the bending moments introduced by the wings and the local loads at the landing gear support structure, which will also require a heavy structure. It must also be noted that, in this configuration, no engine noise shielding is achieved, as there is a direct noise path between the engines and the ground.
Document U.S. Pat. No. 4,365,773 discloses an aircraft having a fuselage and a pair of first wings extending outwardly from the vertical tail, and a pair of second wings extending outwardly from the forward portion of the fuselage, at a lower elevation than the first pair of wings, the pair of wings presenting a double triangle shape or diamond shape along with the aircraft fuselage. A particular merit of this configuration is that the joined wings form a diamond shape in front view, so that they mutually support in bending as well as in torsion, which can result in a lighter wing structure, although a substantially heavier fin and rear fuselage than in a classical configuration can be expected. However, this aircraft configuration, where the rear wing is higher than the forward wing, is also prone to the well known problem of deep stall.
Document U.S. Pat. No. 4,053,125 provides a similar configuration of the joined-wing type as it has been disclosed.
Document U.S. Pat. No. 6,340,134, upon which the preamble of claim 1 is based, discloses an aircraft wing configuration having a high aspect ratio wing generating reduced induced drag. The document discloses a configuration comprising a main wing and a high aspect ratio supplementary wing, these main and supplementary wings being connected by at least two struts. This configuration comprises also a horizontal stabilizer and elevators, necessary to control the aircraft in pitch. The aircraft of U.S. Pat. No. 6,340,134 really functions as a biplane aircraft of the sesquiplane type, where the lower wing is substantially smaller than the top wing and acts mainly as a support for the struts. Although a significant reduction of the induced drag can be expected from this configuration, the friction drag produced by the horizontal stabilizer remains as in the conventional configuration. The use of a structurally efficient lower wing to provide support to the top wing is an enabling factor to have at least a wing of very high aspect ratio without incurring a serious weight penalty. In terms of perceived engine noise, this configuration is also equivalent to the classical aircraft configuration, as the engines are located under the wings, therefore being provided a direct noise path between said engines and the ground. Additionally, the fact that the two wings are substantially parallel may lead to an increased compressible drag in the flight at high speeds due to the aerodynamic interaction of the wings, which forms a flow channel between them.
The present invention is intended to solve above-mentioned disadvantages.